1. Field of the Invention
This invention relates to a radiation image storage panel and more particularly, to a radiation image storage panel comprising a support, a subbing layer and a phoshor layer, superposed in this order.
2. Description of Prior Arts
For obtaining a radiation image, there has been conventionally employed a radiography utilizing a combination of a radiographic film having an emulsion layer containing a photosensitive silver salt material and a radiographic intensifying screen.
As a method replacing the above-described radiography, a radiation image recording and reproducing method utilizing a stimulable phosphor as described, for instance, in U.S. Pat. No. 4,239,968, has been recently paid much attention. In the radiation image recording and reproducing method, a radiation image storage panel comprising a stimulable phosphor (i.e., stimulable phosphor sheet) is used, and the method involves steps of causing the stimulable phosphor of the panel to absorb radiation energy having passed through an object or having radiated from an object; exciting the stimulable phosphor with an electromagnetic wave such as visible light and infrared rays (hereinafter referred to as "stimulating rays") to sequentially release the radiation energy stored in the stimulable phosphor as light emission (stimulated emission); photoelectrically detecting the emitted light to obtain electric signals; and reproducing the radiation image of the object as a visible image from the electric signals.
In the above-described radiation image recording and reproducing method, a radiation image can be obtained with a sufficient amount of information by applying a radiation to the object at considerably smaller dose, as compared with the case of utilizing the conventional radiography. Accordingly, this radiation image recording and reproducing method is of great value especially when the method is used for medical diagnosis.
The radiation image storage panel employed in the above-described radiation image recording and reproducing method has a basic structure comprising a support and a phosphor layer provided on one surface of the support. Further, a transparent film is generally provided on the free surface (surface not facing the support) of the phosphor layer to keep the phosphor layer from chemical deterioration or physical shock.
The phosphor layer comprises a binder and stimulable phosphor particles dispersed therein. The stimulable phosphor emits light (stimulated emission) when excited with stimulating rays after having been exposed to a radiation such as X-rays. Accordingly, the radiation having passed through an object or having radiated from an object is absorbed by the phosphor layer of the radiation image storage panel in proportion to the applied radiation dose, and the radiation image of the object is produced in the radiation image storage panel in the form of a radiation energy-stored image (latent image). The radiation energy-stored image can be released as stimulated emission (light emission) by applying stimulating rays to the panel, for instance, by scanning the panel with stimulating rays. The stimulated emission is then photoelectrically detected to give electric signals, so as to reproduce a visible image from the electric signals. It is desired for the radiation image storage panel employed in the radiation image recording and reproducing method to have a high mechanical strength and high resistance to flexing. That is because the handling of the panel is different from that of the radiographic intensifying screen employed in the conventional radiography, and the panel frequently encounters mechanical shock and receives mechanical force particularly in the course that the panel is irradiated with stimulating rays to read out the radiation energy stored therein.
More in detail, the radiation image storage panel is required to have a high mechanical strength so as not to allow easy separation of the phosphor layer from the support, when the mechanical shock and mechanical force caused by falling or bending of the panel are applied to the panel in use. Since the radiation image storage panel hardly deteriorates upon exposure to a radiation or to an electromagnetic wave ranging from visible light to infrared rays, the panel can be repeatedly employed for a long period of time. Accordingly, the panel subjected to the repeated use is required not to encounter such troubles as the separation between the phosphor layer and support caused by the mechanical shock applied in handling of the panel in a procedure of exposure the panel to a radiation, in a procedure of reproducing a visible image brought about by excitation the panel with an electromagnetic wave after the exposure to the radiation, and in a procedure of erasure of the radiation image remaining in the panel.
The radiation image storage panel has a tendency that the bonding strength between the phosphor layer and the support is decreased as the mixing ratio of the binder to the stimulable phosphor (binder/stimulable phosphor) in the phosphor layer is decresed in order to enhance the sensitivity. The bonding strength therebetween also tends to decrease in the case that the phosphor layer is formed on the support under such conditions that the phosphor particles deposit on the lower side (i.e., the support side), which taking place depending upon the nature of phosphor particles and binder, the coating conditions of the binder solution (coating dispersion), etc.
For enhancing the bonding strength between the phosphor layer and the support which is apt to decrease as described above, it has been known that a subbing layer is provided therebetween. Such subbing layer is formed using a known adhesive agent comprising a synthetic resin. However, when the phosphor layer is formed on the surface of the conventional subbing layer of the support, utilizing the conventional coating procedure, the bonding strength therebetween cannot reach an appropriate level because the material of subbing layer is partly dissolved in the solvent of the coating dispersion for the phosphor layer.
Further, in the conventional procedure of forming a layer of coating dispersion for the phosphor layer, the subbing layer is once swollen by the solvent contained in the coating dispersion and then shrinked, so that cracks are apt to occur on the resulting phosphor layer. Especially in the case that the subbing layer is flexible and the binder of the phosphor layer is relatively rigid, cracks are probably produced on the phosphor layer. Since the occurrence of cracks on the phosphor layer results in deteriorating the quality of an image provided by the panel, it is required to prevent the occurrence of cracks on the phosphor layer.
In the radiation image storage panel having a protective film provided on the phosphor layer, the protective film is usually formed by laminating the surface of the phosphor layer with the film using an adhesive agent under heating and pressure. In the case that the subbing layer is not sufficiently rigid, a portion of the subbing layer is depressed or dislocated in the laminating procedure to bring about uneveness of the thickness thereof or dislocation of the phosphor layer from the support. As a result of such plastic deformation, wrinkles (lamination wrinkles) are likely produced on the surface of the protective film of the resulting panel, or the panel is entirely deformed to have a curved face (namely, curling).